Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 356

9.6.3. Main frames
9.6.3.1. General behavior
Main frames are series of columns and beams that form frames on several floors
and several rows. The column-beam intersections, called nodes, allow loads to be
transmitted between different linear elements. Under permanent and vertical variable
loads, bending moments are typically positive through beam rows and negative near
the supports, whereas the columns are mostly subject to compression normal loads.
Under the effect of a horizontal seismic action, moments vary in a linear way
between nodes within the different elements. Thus, moments are maximum near the
nodes within the columns and beams. Moments change signs during motion because
of the alternating feature of the action. This means columns are subject to bending
moments and shear loads, which are generally more important during an earthquake
than in a normal situation, which explains why they are brittle in the event of an
earthquake.
Beam rows are typically not very sensitive to vertical components of seismic
actions, as they are designed for variable actions that are more intense than the
vertical acceleration on the masses present during an earthquake. This is not true
when the masses have their maximum value (archives room) or when the
acceleration is such that it can reverse the moment sign. On the other hand, the beam
ends near nodes sustain sign inversions of their bending moments, which can
generate traction on the lower sides for which longitudinal reinforcements are
necessary.
Plastic hinges form in areas where bending moments are maximum, near the
nodes. The latter sustain alternating loads transmitted by the linear elements and
expressed by compression and traction inner stresses that can cause damage or node
failure. This should be avoided to preserve the integrity of the structures, so nodes
tend to be over-designed with regard to the adjacent linear elements, according to
the capacity design principles. Most designs try to ensure that plastifications occur
inside the linear element beginnings near the nodes, instead of inside the nodes.
As shown in section 9.3.4, to maximize stability, engineers should avoid having
plastic hinges that form inside the columns during a plastic cycle. However, it
should be noted that it is almost impossible to avoid plastic hinges at the base of
columns to reach the ultimate isostatic condition.
Once the position of plastic columns has been ensured by the application of
capacity design rules, the ductility of the hinges in response to rotation has to be
ensured as well. This is governed by separate local ductility verification standards.